In nature, anadromous fish like salmon live most of their adulthood in seawater, but swim upstream to freshwater for the purpose of breeding. As a result, anadromous fish hatch from their eggs and are born in freshwater. As these fish grow, they swim downstream and gradually adapt to the seawater.
Currently, wild Atlantic salmon are classified as endangered species in multiple areas of their native habitats. Among the reasons for their decline has been man made alterations in freshwater conditions in their native streams that have produced multiple problems with their migration, spawning, smoltification and survival. One problem complicating the effective restoration of wild Atlantic salmon is the lack of a fundamental understanding of how these deleterious environmental conditions effect the salmon's ability to home to freshwater streams from the ocean, interchangeably adapt to freshwater and seawater as well as feed and grow in both salinity environments.
Despite the decline of wild populations, the global aquaculture industry has utilized Atlantic salmon as one of chief fish species for large-scale marine farming operations. At the present time, large scale breeding programs of Atlantic salmon provide for high quality fish used in production by selection of specific traits among them rapid growth, seawater adaptability, flesh quality and taste.
However, fish hatcheries have experienced some difficulty in raising salmon because the window of time in which the pre-adult salmon adapts to seawater (e.g., undergoes smoltification) is short-lived, and can be difficult to pinpoint. As a result, these hatcheries can experience significant morbidity and mortality when transferring salmon from freshwater to seawater. Additionally, many of the salmon that do survive the transfer from freshwater to seawater are stressed, and consequently, experience decreased feeding, and increased susceptibility to disease. Therefore, salmon often do not grow well after they are transferred to seawater.
The aquaculture industry loses millions of dollars each year due to problems it encounters in transferring salmon from freshwater to seawater. Therefore, a need exists to gain a better understanding of the biological processes of salmon that are related to smoltification and adaptation to varying salinities, including seawater. In particular, a need exists to identify genes that play an important role in these areas.